It is possible to build a uniqe building like the Iridium Store or a Power plant more than once on one planet. As soon as the building gets upgraded the lower level version can be built again. So it is possible to have several different power plants sitting next to each other. Is this how it is supposed to be? In GalCiv II you could build only one power plant per planent and if it got upgraded you still could not build another one.
